Knock twice on the bar

In old SF bars, there is an old time tradition of the bartender knocking twice on the bar after taking your money. To be done properly, the bartender will kinda lean towards you, look you in the eye and rap the bar twice with his knuckles. Some (most?) don't even know why they do it or how/when they started it. Story goes that in Gold Rush times, patrons would come in with packets of gold dust as payment for a night of boozing. The barkeeper would "knock it" on the bar twice to make sure it was packed down and not fluffed up with air or other undesirable dust.
Anyone ever notice this, and if so, what's YOUR take on it?

Answers

In my neck of the woods knocking is common, but my take and experience
with it is a little different. The knocks came with a free drink and a
wink that says "Remember, tip me well before you leave". It's a game
that steady customers and professional bartenders knew well. New
bartenders emulate the knocking but don't understand the significance
of the knocks and the knowing wink associated with it that produce the
big payoffs at the end of the night; they'll learn.

I think a poke was more common than a vial. I believe the vials
appeared later with the arrival of Asian opium? (I find them when I dig
around Chinese ruins) and eastern pills, and by then, coins were being
minted in San Francisco and in general circulation. Any gold dust trade
still going on at this time was in the gold fields and everyone knew
the genuine thing. How can you fluff up something that is measured by
weight, not volume? If you toss in enough sand or dirt to affect the
weight significantly, the folly would be immediately detected. Lead
won't work either unless you're colorblind. What's more, I don't think
a poke would ever leave the hand of the owner; it would be like handing
your wallet to the bartender. I don't know, but I'm thinking pinch or
pour might have been the practice of the day and a savvy bartender
would always lick his fingers before dipping in for a pinch.

Micron? A micrometer (micron) is a measurment of lenght equal to one
millionth of a meter. Gold is measured in weight, not length.

P.S. Perris, I like. I can't help but think that Bar owners might have
had a propensity to hire barkeeps with the huge hands, too.

Mike, there was one famous glass vial (well, jar) of gold dust back
in those days. In 1848 Sam Brannan ran through the streets of San
Francisco with a jar of gold dust, screaming at the top of his lungs,
"Gold! Gold in the American River!" Interesting, he had purchased every
pick-axe, every shovel, every pan, every implement of mining and
excavation in the area prior to his publicity stunt and made out like a
bandit. Some accounts say he made 36,000 dollars in a nine weeks, a
truly staggering amount of money in 1848.